The present invention relates generally to a zoom lens system, and more particularly to a compact yet low-cost zoom lens system used with cameras using an electronic image pickup means such as camcorders, digital cameras, supervisory cameras, portable telephones, personal computers, etc.
To achieve size and weight reductions of a consumer-oriented zoom lens system in this field, a specific zoom lens system has been proposed, which comprises four lens groups, i.e., a positive lens group, a negative lens group, a positive lens group and a positive lens group in order from an object side thereof, as typically disclosed in JP-A's 4-43311 and 4-78806. For zooming, the second negative lens group moves on an optical axis while the first and third lens groups remain fixed. The fourth lens group moves to make correction for a fluctuation in the position of an image plane in association with zooming. A further size reduction is achievable by a specific zoom lens system wherein, as typically disclosed in JP-A's 6-94997 and 6-194572, a third lens group moves from an image plane side to an object side of the system during zooming from a wide-angle end to a telephoto end of the system. Such zoom lens systems have a relatively high zoom ratio of the order of 8 to 12. However, these zoom lens systems are still insufficient for zoom lens systems having a lower zoom ratio while weight is placed on ever smaller size and ever lower cost, because of a large number of lenses.
In the zoom lens systems set forth in JP-A's 6-94997 and 6-194572, a great part of the zooming action is shared by the second lens group. To keep an image point substantially constant in this case, the lateral magnification of the second lens group must be in the range of about -1 from the wide-angle end to the telephoto end. At a zoom ratio lower than that, however, the amount of movement of the second lens group can become so small that size reductions can be achieved. It is thus efficient to reduce the space allowed between the first and second lens groups as much as possible for the purpose of size reductions.
To permit the second lens group to have a lateral magnification of about -1 while the space between the first and second lens groups is kept as narrow as possible, however, it is required to increase the power of the first lens group with respect to the second lens group. This then causes an entrance pupil to be located at a farther position where the height of an off-axis ray passing through the first lens group increases, resulting in an increase in the size of the lens system in the first lens group and, hence, an increase in the thickness of the lenses in the first lens group. Further, the curvature of each lens in the first lens group must be increased to make sure of the edge thickness of each lens, resulting in a further increase in the thickness of each lens in the first lens group.